We investigate how the bacterium Legionella pneumophila can exploit human cells and cause a severe pneumonia known as Legionnaires disease. L. pneumophila is commonly found in habitats containing freshwater where it thrives as a natural parasite of unicellular organisms such as amoeba. When inhaled by humans, L. pneumophila can infect alveolar macrophages and cause a respiratory infection that is fatal for up to 30 percent of infected individuals. The ability of this opportunistic pathogen to establish a replication vacuole within infected cells is key to its virulence. The bacterium delivers several proteins, so called effectors, into the host cytosol where they alter host signaling events in order to create an environment supportive for the bacteriums replication cycle. Over the past funding period we have shown that some of the effector proteins alter host membrane transport between the endoplasmic reticulum and the Golgi compartment. The precise mechanism of how transport vesicles are hijacked by L. pneumophila is just beginning to unfold and remains the main focus of this research study. In particular, we analyze L. pneumophila effector proteins that manipulate the activity of the small guanine triphosphatase (GTPase) Rab1, one of the key regulators of this vesicle transport route. Some of these L. pneumophila effectors appear to be molecular mimics of human proteins that catalyze Rab1 activation through GDP/GTP exchange and Rab1 deactivation through hydrolysis of GTP. In addition, we discovered a second regulator circuit unique to L. pneumophila in which bacterial effector proteins attach or remove a post-translational modification to Rab1 in order to stabilize its activation state. This novel concept has not been described in any other virulence system and is the focus of our ongoing studies. The results obtained here will provide important insight into the virulence strategies of L. pneumophila and create the foundation for the development of better therapeutic approaches to treat Legionnaires disease and related illnesses.